The duplex silicon (Si)-silicon carbide (SiC) ceramic, hereinafter designated as Si-SiC, shows excellent physico-chemical properties like high oxidation, corrosion and thermal shock resistance, superb high temperature strength and toughness together with other related properties, e.g., Young's modulus, hardness, etc., and therefore, is of great technological importance. These properties can be exploited during the applications of the material in the fields of kiln furniture particularly for heating sanitary wares, saggers, and crucibles for calcining fluorescent powder in fluorescent lamp industries, high speed and high service life domestic and industrial burners, rocket nozzles, recuperators for waste heat utilization in industrial furnaces, heat-exchangers for indirectly fired open/combined cycle gas turbines to generate electric power, ceramic seals for pumps handling corrosive fluids, wear inserts, etc.
SiC ceramics are fabricated from synthetic powders by hot pressing/hot isostatic pressing, pressureless sintering, polymer pyrolysis, chemical vapor deposition, liquid silicon infiltration processing (LSIP) or reaction bonding/reaction sintering. Reference is made to the Ph.D. Thesis (“Reaction sintering of silicon carbide” by O. P. Chakrabarti, University of Calcutta, 1998) and also to an article (“Reactive infiltration of Si—Mo alloyed melt into carbonaceous perform of SiC”, by O. P. Chakrabarti and P. K. Das published in the J. Am. Ceram. Soc., 83[6]1548-1550 (2000)), wherein the authors present a detailed review of the different processing routes of SiC ceramics with particular emphasis on the Si or alloyed Si melt infiltration processing of Si—SiC ceramics.
The drawbacks of the referred works in the mentioned reviews, are requirement of expensive synthetic raw powders, complex preform making and difficult machinability of the finished product etc. However, synthesis of ceramic materials from naturally grown plant structures has recently received interests. Plants often possess natural composite structures and exhibit high mechanical strength, low density, high stiffness, elasticity and drainage tolerance. These advantages are because of their hierarchically built anatomy developed and optimized in a long-term genetic evolutionary process. There is a possibility of producing novel ceramic materials with a unique microstructure pseudomorphous to that of naturally grown plant structures. The bio-structure derived ceramic materials would have tailorable properties with numerous potential applications. Synthesis of SiC ceramic from naturally grown plant structures has recently received interest among the material scientists.
References is made (i) to U.S. Pat. No. 3,754,076, titled “Production of silicon carbide from rice husk” by I. Cutler, 21 Aug. 1973) and also to an article (“Formation of silicon carbide from rice hulls” by J. G. Lee and I. B. Cutler, published in the Am. Ceram. Soc. Bull., 54[2] 195-98 (1975)) wherein the authors reported the production of SiC whiskers from thermal decomposition of rice hulls, a waste product, (ii) to an article (“Formation and structure of silicon carbide whiskers from rice hulls” by N. K. Sharma, W. S. Williams and A. Zangvil, published in the J. Am. Ceram. Soc., 67[11] 715-720 (1984)) wherein the authors informed of a study relating to the distribution of silicon (Si) in hulls of common brown Indian rice, carried out to aid in understanding the formation of SiC whiskers and also of ultrafine β-SiC particles by thermal decomposition, (iii) to an article (“Growth of β-SiC whiskers by LVS process”, by J. V. Milewski, F. D. Gac, J. J. Petrovic and S. R. Skaggs, published in the J. Mater. Sci., 20, 1160-1166 (1985)) wherein the authors described a vapor-liquid-solid process for the formation of the SiC whisker by the catalyzed thermal decomposition of rice husk, (iv) to Indian Patent 172941 titled “A process for the production of silicon carbide fibers (β-form) from rice husk” by M. Patel, C. B. Raiu, A. K. Ray and A. Karera, 8 Jan. 1994) and also to an article (“Effect of thermal and chemical treatments in rice husk” by M. Patel, A. Karera and P. Prasanna, published in the J. Mater. Sci, 22, 2457-2464(1987)) wherein the authors described the formation of SiC whiskers by carbonizing coked rice husk pretreated with hydroxides of metal catalysts, (v) to an article (“SiC whiskers from rice husks: role of catalysts” by M. Patel and A. Karera, published in the J. Mater. Sci. Letts., 8, 955-956 (1989)) wherein the authors describe the formation of SiC whiskers by carbonizing rice husks without added catalyst, (vi) to an article (“Direct pyrolysis of raw rice husks for maximization of SiC whisker formation” by R. V. Krishnarao and M. M. Godkhindi, M. Chakraborty and P. G. Mukunda, published in the J. Am. Ceram. Soc., 74, 2869-2875 (1991)) wherein the authors described the formation of SiC whiskers by pyrolysis of raw rice husks without any catalyst or precooking, (vii) to an article (“Conversion of raw rice husks to SiC by pyrolysis in nitrogen atmosphere” by R. V. Krishnarao, Y. R. Mahajan and T. J. Kumar, published in the J. Euro. Ceram. Soc., 18, 147-152 (1998)) wherein the authors describe the formation of SiC whiskers by pyrolysis of raw rice husks without precooking, (viii) to an article (“Synthesis and characterization of SiC whiskers from coconut shells” by A. Selvam, N. G. Nairand P. Singh, published in the J. Mater. Sci. Lett., 17, 57-60 (1998)), wherein he authors described a method of producing SiC whiskers by pyrolyzing raw coconut shells soaked with 10% ferrous chloride solution followed by drying. The drawbacks of the referred works are high pyrolysing temperature, poor yield, uneconomical conversion because of the requirement of very large pyrolysing reactor for handling voluminous assemblage of rice husk and unshaped whisker/powder which needs further processing for making shapes of controlled microstructure.
Reference is made to an article (“Biomimetic process for producing SiC wood” by T. Ota, M. Takahashi, T. Hibi, M. Ozawa, S. Suzuki and Y. Hikichi, published in the J. Am. Ceram. Soc., 78[12] 3409-11 (1995)), wherein the authors describe a method of making SiC in wood like structure by infiltrating tetra ethyl ortho silicate (TEOS) into a piece of oak charcoal followed by hydrolysis by treatment with ammoniacal solution and by heat treating the charcoal containing SiO2 gel. The drawbacks of the referred work are requirement of multiple TEOS infiltration followed by subsequent firing, porous final product with unconverted carbon in the structure and incomplete conversion of C to SiC.
Reference is also made to a two-part article (“Biomorphic cellular silicon carbide ceramics from wood: I. Processing and Microstructure” by P. Greil, T. Lifka and A. Kaindl published in the J. Euro. Ceram. Soc. 18, 1961-1973 (1998) and “Biomorphic cellular silicon carbide ceramics from wood: II. Mechanical Properties” by P. Greil, T. Lifka and A. Kaindl published in the J. Euro. Ceram. Soc. 18, 1975-1983 (1998)) wherein the authors describe a method of making cellular SiC ceramics with anisotropic pore structures by infiltrating liquid silicon at 1600° C. for 4 hours without pressure into carbonized woody (beech, oak, maple, pine, balsa and ebony) specimens. Long infiltration cycle, lower product density and microstructure of the final product randomly varying with precursor wood without correlation, constitute the main drawbacks of the referred work.
Reference is made to an article (“Silicon/silicon carbide composites fabricated by infiltration of a silicon melt into charcoal” by D. W. Shin, S. S. Park, Y. H. Choa and K. Nihara, published in the J. Am. Ceram. Soc., 82[11] 3251-53 (1999)) wherein the authors describe a method of making dense Si/SiC composite by infiltrating liquid Si (obtained by melting powder Si) at 1700° C. under vacuum into porous commercial oak charcoal specimen. The drawbacks of the referred work are higher infiltration temperature and difficulty in infiltration procedure likely to be arising out of usage of powdered silicon under vacuum.
Reference is also made to U.S. Pat. No. 6,124,028, titled “Carbonized wood and materials formed thereform” by Denis C. Nagle and Christopher E. Byrne, 26 Sep. 2000) wherein the authors described a method of making SiC ceramic by reacting stoichiometric to greater than stoichiometric quantity of silicon at 1500° C. for 10 minutes to 2 hours in flowing argon, with carbonized wood specimens (red oak, balsa, basswood, maple, white pine, red wood). The drawbacks of the referred work are incomplete infiltration, difficulty in unloading of the final product because of sticking problem and absence of correlation between the product microstructure and structure of the precursor wood.
Reference is made to an article (“Environment conscious ceramics (Ecoceramics)” by M. Singh published in Ceram. Eng. Sci. Proc., 21[4] 39-44 (2000)), wherein the author described a method of fabricating SiC-based ceramics by reactive infiltration by molten silicon at 1450° C. for 30 minutes into pyrolyzed wood specimens (Brazilian Rosewood, Africian Zebra, Ceylon Stainwood, Africian Bubinga, Pau Lope, Australian Jarrah and Indian Mango wood). The drawbacks of the referred work are long infiltration cycles required for processing the ceramic and product microstructures that vary according to the parent plant structure but remain un-correlated.
A wide variety of plants are used for making different varieties of SiC ceramics. Plant shells make possible mostly the production of SiC whiskers or fibers, but the manufacturing processes are just short of being commercially viable. Biological preforms from various naturally grown plant stems including soft woods and hard woods are employed for producing bulk SiC ceramics. In view of the variations in dimensions, compositions and morphology of the naturally grown plant structures, the shape and composition of the bulk SiC produced may vary significantly.
However no studies have been found relating to the selection of the suitability of plant structure in terms of its anatomical and structural features, which after having been transformed to carbonaceous preform, helps in faster infiltration and reaction of liquid through porous contour to get converted to SiC. No study has been done, which relates to overcome the difficulties of withdrawing infiltrated specimen form contacting liquid after the completion of the infiltration and reaction. No study is available which avoids the costly post-infiltration treatment for removal of adhered melt from product surface The present invention has been developed in view of the foregoing and other deficiencies of the prior art.